1. Field of the Invention
The present invention relates to a method for carrying out a soft handoff in a cellular mobile communication system, and a system and a base transceiver station for implementing the same.
2. Description of the Related Art
In a cellular mobile communication system, radio channel switching control performed when a mobile unit (car telephone, portable telephone, etc.) moves from one cell into another cell is known as handoff. One handoff control method is the so-called soft handoff technique.
While the process of ordinary handoff (sometimes called hard handoff as opposed to soft handoff) involves disconnecting the radio link in the first cell upon initiation of a handoff and then establishing a radio link with the second cell, the soft handoff process is characterized by establishing a radio link with the second cell while maintaining the radio link with the first cell (that is, radio links to two (or more) cells are operating simultaneously). This handoff technique is often used, among others, in code division multiple access (CDMA) systems (Japanese Unexamined Patent Publication No. 4-502845). The soft handoff technique offers the following advantages (Japanese Patent Unexamined Publication No. 4-502845).
(1) Uninterrupted handoff is possible.
Unlike hard handoff, no interruptions occur during a conversation, etc. when a handoff takes place.
(2) Inter-cell diversity is possible.
Generally, radio communication quality deteriorates near cell boundaries, but maintaining simultaneous communications with more than one cell by soft handoff serves to achieve inter-cell diversity, providing increased resistance to shadowing and fading.
Generally, the advantage (1) is receiving attention as the strong point of soft handoff, but in practice, as is apparent from the analysis conducted by A. J. Viterbi et al. (A. J. Viterbi and A. M. Viterbi, "Other-Cell Interface in Cellular Power-Controlled CDMA," IEEE Trans. on Commun., Vol. 42, No. 2/3/4 (1994)), soft handoff is an essential technique for CDMA where transmission power control is performed, since, without soft handoff, interference with other cells would become extremely large, substantially reducing the number of available radio channels (the number of subscribers that can be served). If "improving service quality" as exemplified by the advantage (1) were a primary consideration, whether to use or not use soft handoff could be determined at discretion, but the fact that soft handoff is an essential technique means that soft handoff must be carried out as simply as possible and at low cost.
Previous systems have had the following problems since soft handoff and inter-cell diversity are both controlled centrally at a mobile telephone switching office (MTSO) (or a base station controller (BSC); an MTSO or a BSC can be called a concentrator).
(1) Since a soft handoff to a base transceiver station (BTS) served by a different concentrator is not possible,.if a handoff to a BTS served by a different concentrator occurs, quality deterioration and inter-cell interference increase because of a momentary interruption and an inability to provide inter-cell diversity, leading to substantial degradation in system performance.
(2) The system, and particularly the concentrator, becomes extremely complex and expensive.
That is, to obtain a sufficient soft handoff effect, and to avoid a handoff involving different concentrators such as described in (1) as much as possible, a large number of BTSs must be accommodated under one concentrator. Furthermore, according to A. J. Viterbi et al., during the execution of a soft handoff each mobile station (MS) needs to be connected simultaneously to at least two BTSs, and for inter-cell diversity, three to four BTSs. This greatly increases the processing capability demanded of the concentrator. The concentrator thus increases in size, complexity, and cost. Additionally, cost reductions by mass production cannot be expected in the case of concentrators since the number of concentrators installed is small. Moreover, since a failure of one concentrator could lead to communication failures of the large number of BTSs (cells) under the control of that concentrator, high reliability is demanded, pushing the cost of the concentrator further upward.